A lithium-ion or sodium-ion battery comprises at least one negative electrode, one positive electrode, one separator and one electrolyte. The electrolyte is composed of a lithium or sodium salt dissolved in a solvent, which is generally a mixture of organic carbonates, in order to have a good compromise between the viscosity and the dielectric constant.
The most widely used salts include lithium hexafluorophosphate (LiPF6), which has many of the numerous qualities required but exhibits the disadvantage of decomposing in the form of hydrogen fluoride gas. This presents safety problems, in particular in the context of the impending use of lithium-ion batteries in specific vehicles.
The prerequisite for having an electrolyte salt is good chemical dissociation between the cation and the anion, which implies a negative charge on the anion which is delocalized or reduced by withdrawing effects.
Salts based on the withdrawing effect have thus been developed, such as LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) and LiFSI (lithium bis(fluorosulfonyl)imide).
Other salts, this time based on the delocalization of the charge, have also been developed, such as LiTDI (lithium 4,5-dicyano-2-(trifluoromethyl)imidazolide), as is taught in the document WO 2010/023413. However, the latter exhibit ionic conductivities which are lower than those mentioned above.
The applicant company has discovered that the presence of a second aromatic ring makes it possible to increase the delocalization of the negative charge and to thus increase this ionic conductivity.